1. Field of the Invention
This invention relates to a method of controlling a sulfidation reaction of a heavy metal such as zinc in a solution by using hydrogen sulfide, and particularly to a method of sulfidation reaction to selectively remove zinc from a solution.
2. Description of the Related Art
In the hydrometallurgy method or waste water treatment method of metals, the method of fixation of the heavy metal component in a solution by a sulfidation reaction using hydrogen sulfide has been widely used as the main fixation method. For examples technology to recover sulfides of nickel and cobalt from a solution containing nickel and cobalt is disclosed in JP Patent publications of Tokukai Sho 47-2555 and Tokukai Hei 6-116660. Incidentally, there is no description on movement of zinc in the publications.
It is said that control of this sulfidation reaction is generally difficult. Oxidation-reduction potential (ORP) and pH must be controlled to obtain proper sulfidation reaction. This sulfidation reaction is a reaction that generates fine sulfide slime, so the surface of the controller such as an electrode is covered by the reaction product and it becomes difficult to accurately measure the oxidation-reduction potential (ORP) or pH, or it becomes difficult to stably control the amount of flow of hydrogen sulfide due to blockage of the intake pipe for hydrogen sulfide gas which is injected to control the ORP and pH.
In order to solve the problem of controlling the reaction as described above, measures are taken such as industrially performing a sampling of the reaction solution and measuring the ORP or pH using an external system, or by blowing a constant flow of inert gas such as nitrogen gas into the piping. However, in the case of the former, a problem occurs in that a time lag occurs due to the time required for taking the sampling, so this it is not suitable when performing fine reaction control; and in the case of the latter, a problem occurs in that the reaction cost increases due to constantly having to blow in the inert gas.
Also, in order to adopt these methods, there is a problem in that sampling equipment, or equipment for blowing the inert gas is necessary.
On the other hand, the fixing method by way of sulfidation reaction is also used for selectively removing an impurity of zinc from the solution containing cobalt, nickel, ion, manganese and othser metals. A zinc removal step using hydrogen sulfide gas is disclosed in JP Patent publication Tokukai Sho 63-45130.
When removing zinc, specifically zinc sulfide, from such a solution, since the acidity limitation value for precipitating MnS, CoS, NiS and FeS from the respective solutions is substantially different from that for ZnS (for example, HCl 0.02N for ZnS while HCl 0.001N for CoS and NiS), it is expected that ZnS is first easily separated in the lower pH range. However, ZnS is inclined to induce co-precipitation of NiS and FeS, so that it is difficult to selectively separate only zinc from the reaction solution. Therefore, JP Patent publication Tokukai Sho 63-45130 discloses a method for zinc removal where e.g. calcium carbonate is added to the solution to neutralize the acid produced through reaction to keep the pH in the solution to a proper value for precipitation of zinc sulfide.
However, in this method, since chemicals are added for pH control during injection of hydrogen sulfide gas, reaction between the pH control agent and nickel and cobalt is locally caused to produce hydroxides. This is a problem because of loss in object metals such as nickel in addition to the problems as mentioned above.
Due to the problems mentioned above, there was a need to develop a sulfidation reaction control method that makes it possible to easily perform fine reaction control.